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DGPS-Differential Global Positioning System Basics

DGPS avionic system is a approach landing navigational aids and it stands for Differential Global Positioning System. The Ground system used to increase GPS accuracy. In fact, DGPS is typically found in airports in order to bring a more precise aircraft position fix during the approach landing phases. As for the aircraft, it must be equipped with a GPS-Rx so that signals from both satellite and DGPS would be observed to correct position errors.

DGPS,Differential Global Positioning System

On the ground DGPS Tx and Rx system operates with following specifications:
•  Rx uses UHF band which is used to calculate DGPS position fix.
•  Tx uses LF/MF band(283.5 to 325 KHz), it is used to transmit correction signal to the Aircraft.
•  DGPS is stationary, therefore its actual LAT,LON,ALT is well known. This is used to transform it to XDO, YDO and ZDO.
•  Similar to GPS, here also four satellites are needed to obtain a 3D position fix for the DGPS. This is identified as XDO, YDO and ZDO.
•  The DGPS then takes the difference that exists between the actual known position and the calculated position obtained using GPS SATs.
δ = Calculated position - Actual Position
•  Ideally, δ should be roughly zero; however, most of the time this is not the case, and therefore the δ information is transmitted to the airborne GPS-Rx in the form of a correction signal.

In the Aircraft GPS receive system has following features.
•  It uses UHF band to calculate Aircraft position fix and LF/MF band to observe correction signal from DGPS.
•  Range of operation is about 370 Km.
•  Again, the A/C obtains its 3D position fix using the 4-SATs.
•  Then, the GPS receiver detects correction signal that contains the δ information and performs the following correction.
Actaul position = Calculated position - δ

Advantages

•  GPS is quite accurate; however, using DGPS pushes its accuracy even further.
•  GPS/DGPS makes A/L guidance every precise as oppose to ILS and MLS.

Disadvantages

•  Most of the errors are either completely eliminated or made negligible after using DGPS; however, atmospheric (i.e. Ionosphere & Troposphere) and Rx-based (i.e. Rx-Noise & Multipath) errors would still exist. DGPS error sources are satellite clocks, orbital error, ionosphere, troposphere, rx noise and multipath.

•  The coverage area to take advantage of DGPS is limited.
•  To ensure greater coverage area more DGPS stations need to be added.
•  The position accuracy degrades as the separation between DGPS and aircraft GPS-Rx increases.

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